The present invention relates to electrical generators used in gas turbine engines. More particularly, the present invention relates to a generator with an oil system separated from the generator motor.
Gas turbine engines, such as turbofans, commonly include accessory systems, which may be distinguished from the principle engine components, for example, the fan, compressors, combustor, and turbines. One such accessory system is the generator, sometimes referred to as the starter generator, integrated drive generator, variable frequency drive generator, or variable frequency generator. The generator is an electrical system used to power various electrical systems in the engine, as well as the onboard electrical systems of the plane to which the engine is attached. The generator is commonly attached to and driven by an accessory gearbox, which in turn is driven by one of the engine main shafts, for example, the compressor shaft. The generator and accessory gearbox are commonly mounted radially outward from the main axis of the engine, aft of the fan and low pressure compressor section, and inside a nacelle around which working medium gas is driven by the fan section to produce thrust.
Generators create several challenges in gas turbine engine design. In particular, the size and arrangement of prior generators in gas turbine engines has degraded engine efficiency by necessitating a radially outward bulge in the nacelle surrounding the generator. The nacelle shape is important to engine efficiency, as the nacelle defines the aerodynamic surface across which working medium gas is driven from the fan section to produce thrust. The bulge in the nacelle shape surrounding the generator degrades efficiency by acting to remove energy from the working medium gas as it travels over the bulged surface. In high bypass turbofan engines, the nacelle shape is particularly important to engine efficiency, because a large percentage, for example 75%, of the thrust used to propel the engine is produced by the fan section. Additionally, the generator commonly remains the same size regardless of the engine size, which makes the relative size of the generator with respect to the engine increase as the engine gets smaller. As the relative size of the generator increases, so does the relative size of the bulge in the nacelle surrounding the generator. Therefore, the performance penalty resulting from the sub-optimum nacelle shape surrounding the generator increases as the engines size decreases.